Boils

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Color-adjusted photo (the original is pretty much rusty orange) of the inside end wall of one of our boilers. This is inside the water tank of the boiler.

The key question here is how much scale buildup (the hard crusty crud) is on the inside of the tank. Scale deposits are mostly calcium from the water, plus iron that has eroded off the insides of our steam pipes and equipment.

Scale is a great insulator. So more scale means you have to burn a lot more gas to get the heat through to the water to boil it.

You see what looks like a lot of scale here. The good news is that we’re making headway. There used to be more.

Tubes, inside the same tank, coated in scale. This shows the true color of the scale: very rusty. That tells us we have a lot of iron in our water system, which means too much free oxygen in the steam, which means we need to treat the water we feed into our boilers with more oxygen scavengers. (Aren’t you glad you asked?)

The oxygen essentially causes the insides of all our steam pipes and steam-heated equipment to rust at a faster rate. So we end up with rusty scale inside the boiler (where it collects) and leaks and holes in pipes and throughout the steam system (which are annoying to repair and cause production delays).

We check and photograph these tubes every time we have a reason to open the boiler, typically about once a year. Previous measurements indicate about 1/8 inch of scale on the outsides of these tubes. I didn’t measure it this time, but it hadn’t changed much.

Normally you might expect the scale to be more white in color, which would suggest more calcium in it, less iron. You use different solutions to knock back scale with a lot of calcium in it. So it’s good to have a look from time to time to visually judge what you’re trying to fix.

The heat in this boiler comes from inside the tubes. A huge gas flame feeds all the tubes at once. Again, the more scale on these tubes, the hotter that flame has to burn in order to boil the water that normally fills this tank.

Because in this boiler the flame goes through the tubes, it is called a fire-tube boiler.

Knew you’d want to know that.

This, on the other hand, is a water-tube boiler. In this boiler, the tubes (three sets of curving tubes in this picture) are filled with water. The fire comes from underneath the tubes. (You can see the burners—long, flat rectangles under the tubes.) These tubes are much smaller than in the fire-tube boiler, but that maximizes the surface area being heated by the fire at any given time, so this boiler tends to respond faster when you turn it on.

As the water boils, it makes its way up into the big tank at the top of the picture, behind the manifolds at the top of each tube set.

Both of these boilers were modified a couple of years ago to meet more stringent California rules on emissions from natural-gas burners.

The problem in the water-tube boiler is the same as in the fire-tube boiler, but in reverse: The scale builds up on the water side of the process, which in this case is inside the tubes instead of on the outside. Same scale, same source, same end result: You have to put a lot more flame to these tubes to bring the water up to a boil.

The tube set on the right in the picture above this one has been cooked hotter and longer than the other tube sets; you can see the difference in the metal color. Ultimately that’s not good for the metal. (Also notice that the tube set on the left has had a nasty leakage problem down the front of it. The white deposits are what’s left behind as the water evaporated in the fire.)

Overall, I would say we had less scale inside the tubes this time than when I’ve checked them in the past. The boiler inspector who came around today was satisfied with what he saw.

Old and new, showing progress.

The new tubes in this picture (we’re back in the fire-tube boiler) are nice and clean. We put brand-new tubes into this part of the boiler several months ago. You can see there’s no scale on the outside of the tubes.

The big tube next to them wasn’t replaced. See the gray patches of metal that show through the red scale on this tube? Pictures from two years ago show solid scale covering this tube. So our water-treatment plan is making a difference.

Now you know all there is to know about boilers. Next week’s lesson: adhesive systems and the carriers that go with them.